Fix QuickView windows not staying on top on some linux systems

Fixes #1253922 [quickview should be pinned on top of main GUI](https://bugs.launchpad.net/calibre/+bug/1253922)

Merge branch 'master' of https://github.com/cbhaley/calibre
Duplicates dialog, remember window size. Fixes #1254332 [[Enhancement] enlarge the window reporting duplicates after importing new books.](https://bugs.launchpad.net/calibre/+bug/1254332)
Get rid of the workaround for mtpz debug message by compiling libmtp with --disable-mtpz
Make the smart device cache return instances of USBMS.Book instead of Book.Metadata.